The present invention relates to a method for the preparation of a body of an austempered ductile cast iron or, more particularly, to a unique method for the preparation of a body of an austempered ductile cast iron having a gradient o the mechanical properties within the body along with excellent toughness and strength suitable as a substitute material for a forged steel.
It is an established process highlighted in recent years to prepare an austempered ductile cast iron, referred to as an ADI hereinafter, having good mechanical properties suitable as a substitute material for conventional forged steels by subjecting a body of a nodular graphite cast iron to an austempering heat treatment.
The austempering heat treatment of a nodular graphite cast iron to obtain an ADI is carried out usually at a temperature in the range from 250.degree. to 450.degree. C. while the mechanical properties of the thus obtained ADI largely depend on the treatment temperature so that various grades of ADI materials having mechanical properties to meet the requirements in a particular application can be obtained by controlling the temperature in the austempering treatment. For example, ADI materials are classified in Japanese Industrial Standards (JIS) into three grades including a high-toughness grade of FCD900A, a high-hardness grade of FCD1200A and a grade of FCD1000A having intermediate characteristics between the former two.
Since it is a usual practice that the temperature distribution in the atmosphere or bath for a heat treatment is controlled to be as uniform as possible, any body of an ADI material naturally has a substantially uniform distribution of the mechanical properties such as toughness and strength throughout the body determined by the selected temperature for the austempering heat treatment. Needless to say, it is not always the optimum that a structural member has uniform mechanical properties throughout the body or in all of the directions or points within the body. Rather, it would be sometimes desirable that a structural member has a gradient in the mechanical properties each point having properties differing from the other points so that the requirements for the structural member, which may vary from point to point, can be satisfied most satisfactorily. For example, a cylinder piston of an internal combustion engine is required to have a high hardness on the surface in a sliding contact with the cylinder walls while the upper part of the piston member to be connected with the piston rod should desirably have a high tensile strength and toughness. Nevertheless, no prior art method is known for the preparation of a body of an ADI having a graident of the mechanical properties within the body.